Screening thyrometabolic disorders using total serum thyroxine level.
The results of in-vitro thyroid function tests and clinical details of 1,517 patients are reviewed, and the total serum thyroxine levels and the free thyroxine indices are compared in terms of their diagnostic value. Only 3-7% of the patients investigated had a total serum thyroxine level (T4) that did not correlate with the free thyroxine index. We believe that the evidence is strong enough to suggest that estimation of the T4 is an adequate screening test for thyrometabolic disease, and that performing the extra test (T3 resin uptake) in order to estimate the free thyroxine index (FTI) cannot be justified in terms of cost-benefit. We suggest that the FTI is indicated only in those cases in which the T4 is outside the 80% limit range. A simple scheme for the investigation of thyrometabolic disorders is presented.